Crimping, which is a special kind of beading, is a method for joining parts, in particular a wire with a connector (often having the shape of a plug), by plastic deformation. The resulting permanent joint provides good electrical and mechanical stability and is thus a suitable alternative to other connecting methods such as welding or soldering. Hence, common fields of application for crimping are electric devices (e.g. for telecommunication, electrical equipment for vehicles, etc.). The shape of a crimp should be exactly adapted to the wire so as to provide for a predetermined deformation of the same. Crimping usually is done by a crimping gripper or a crimping press.
According to prior art, the force acting during the crimping process can be measured to monitor and/or ensure a constant quality of crimp connections manufactured by a crimping press. For example, pressure sensors are utilized for this reason, which measure the force between the frame and the die (14) and/or the drive and the plunger (15) (see FIG. 5). A further possibility is to evaluate the deformation of the frame of a crimping press.
For example, U.S. Pat. No. 5,841,675 A discloses a method of monitoring the quality of crimping process. To ensure a particular quality, the peak factor, which is defined as crimp work divided by peak force, is determined. The method includes setting the boundaries based upon the mean and standard deviation of a number of learned samples.
Furthermore, U.S. Pat. No. 6,418,769 B1 discloses a method of monitoring a crimping process, wherein a force stroke progression occurring during crimping is measured and compared to a nominal force stroke progression. The evaluation is done above a particular threshold value.
In addition, EP 1 243 932 A2 discloses a method of monitoring a crimping process, wherein a force time progression occurring during crimping is measured, the crimping work is calculated, said progression is separated in segments and the actual work of a segment is compared to a nominal work.
Moreover, U.S. Pat. No. 5,937,505 A, discloses a method of monitoring a crimping process, wherein a force stroke progression occurring during crimping is measured and checked whether it is within a reference region. Statistical theory is utilized to develop a continuous band of allowable variation in the progression.
Furthermore, EP 0 460 441 B1 discloses a method of monitoring a crimping process, wherein a force stroke progression occurring during crimping is measured. A group of data element pairs is selected from said progression in an interesting region. This group of data element pairs is analyzed and compared to a standard group of pairs taken during a known high quality crimp cycle to determine the quality of the present crimped connection.
Finally, EP 0 730 326 A2 discloses a method of evaluating a crimped electrical connection, which measures the crimping force over a range of positions of the crimping apparatus ram and derives a statistical envelope of acceptable forces. Each crimp is measured and the force measurements are compared against that envelope to determine the acceptability of the crimp. Acceptable crimps are then further evaluated to determine whether their data should be added to the data base.
However, despite all measures, which have been taken to make the decision whether a crimp connection is qualified a good (passed) or bad (failed) “fuzzy”, meaning allowing some variation of the crimps, there is still room for improvement.